Introduction – What Is pH ?

When growing organic marijuana or medical marijuana the pH level of your soil is a main factor in determining whether you have a successful crop or not. The importance of pH and marijuana can not be overstated.

What is pH ? In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline. Pure water has a pH very close to 7.

Soil pH is a measure of the acidity or basicity in soils. Soil pH is considered a major factor in soils as it controls many chemical processes that take place. It specifically affects plant nutrient availability by controlling the chemical forms of the nutrient. The optimum pH range for most plants including marijuana is between 6.2 and 7.0, however many plants have adapted to thrive at pH values outside this range. Mycorrhizal fungus is what enables plants to survive outside of this pH range. More on that later.

Why pH is so Important !

Nutrient availability in relation to soil pH Nutrients needed in large amounts by plants are called macro-nutrients and include nitrogen, phosphorus , potassium, calcium, magnesium and sulfur. Elements that plants need in trace amounts are called trace nutrients or micro-nutrients. These micro-nutrients are not major components of plant tissue but are essential for growth. They include iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), cobalt (Co), molybdenum (Mo), and boron (Bo). Both macro-nutrient and micro-nutrient availability are affected by soil pH. In slightly to moderately alkaline soils, molybdenum and macro-nutrient availability is increased except for phosphorus. Phosphorus along with, Fe, Mn, Zn Cu, and Co levels are reduced and may adversely affect plant growth. In acidic soils, micro-nutrient availability (except for Mo and Bo) is increased. Nitrogen is supplied as ammonium or nitrate in fertilizer amendments, and dissolved (soluble) nitrogen will have the highest concentrations in soil with pH 6–8. Concentrations of available nitrogen are less sensitive to pH than concentration of available phosphorus. In order for phosphorus to be available for plants, soil pH needs to be in the range 6.0 and 7.5. If pH is lower than 6, phosphorus starts forming insoluble compounds with iron and aluminium and if pH is higher than 7.5 phosphorus starts forming insoluble compounds with calcium (Ca). Most nutrient deficiencies can be avoided between a pH range of 6 to 7, provided that soil minerals and organic matter contain the essential nutrients to begin with. PH levels are crucial for a successful crop of organic marijuana.

Changing soil pH

Increasing pH of acidic soil! The most common amendment to increase soil pH is lime, usually in the form of finely ground agricultural lime. The amount of lime needed to change pH is determined by how finely it is ground and the buffering capacity of the soil. Buffering capacity of soils is determined by the clay content of the soil, the type of clay and the amount of organic matter present. Soils with high clay content will have a higher buffering capacity than soils with little clay. Soils with high organic matter will also have a higher buffering capacity than those with low organic matter. Soils with high buffering capacity require a greater amount of lime to be added than a soil with a lower buffering capacity for the same incremental change in pH. Other amendments that can be used to increase the pH of soil include wood ash, and oyster shells, but lime is more convenient and a more accurate way of increasing pH.

What type of lime to use ? When growing organic marijuana what lime you use is important. Although agricultural lime is most commonly used to increase soil pH it is not the best choice as it may not be organic. If the lime is hydrated it is inorganic, this is because of the process implemented to create it. Always read the label to see if it is hydrated or not. Some agricultural lime is hydrated and some is not. The best lime to use is Dolomitic Lime also refereed to as dolomite lime. Dolomitic lime comes from mined dolomitic limestone. The limestone when processed produces a powdered mixture of lime and 35 to 40 percent magnesium oxide. This mined limestone is totally organic containing calcium and magnesium making it the best choice. The other reason for using dolomitic lime is that it will never raise the pH of your soil above 7which is neutral. This holds true no matter how much dolomitic lime you use. The best way to use dolomitic lime is to mix it with your organic soil before you plant in it. I use 4 tablespoons of dolomitic lime to 10 gallons of soil. If you mix dolomitic lime in your soil you will almost never have to worry about pH during your entire grow from beginning to end. The dolomitic lime buffers your soil to keep pH in check. I have never had a pH problem if I mixed dolomitic lime into my soil prior to planting.

Decreasing pH of alkaline soil ! Iron sulphates or aluminium sulphate as well as sulfur reduce pH through the formation of sulfuric acid. Many fertilizers such as urea, urea phosphate, ammonium nitrate, ammonium phosphates, ammonium sulphate and mono-potassium phosphate fertilizers also decreases the pH of soils. Most if not all organic liquid fertilizers will also lower the pH of soils. Organic matter in the form of plant litter, compost, manure and peat will decrease soil pH through decomposition. This is all a good thing because we want our organic soil to be slightly acidic for growing marijuana. When growing organic marijuana it is rare to have alkaline soil because of this decomposition of the organic matter in our soil. If for some reason you find your soil is alkaline, one way to lower the pH is with “micronized” sulfur. The reason to use micronized sulfur is that it dissolves in water easier than regular powdered sulfur. Micronized just means it has been ground into smaller particles to dissolve easier.

Your soil mix will determine the pH. What is the main ingredient of your soil ? If your main ingredient is “peat” your soil will be highly acidic. If you main ingredient is “compost”, your soil will be slightly acidic. If your main ingredient is “coco fiber”, your soil will be neutral. Any other ingredients you add will have an effect on the pH of that soil. This is the reason we add dolomitic lime to the soil mix, to stabilize pH.

How Do You Know When You Need To Worry or Adjust Soil pH ?

If you incorporated dolomite lime in your soil mix you will almost never have to worry. I say almost because there are always unusual circumstances that can throw pH off. If your water supplies pH has drastically changed for some reason it may cause a problem. A forest fire in your area or near your water source can drastically alter water pH ! A flood in or near your area can also alter pH drastically !A drastic pH change in the water you are using may or may not affect your plants. I say this from experience. A forest fire in my area drastically changed the pH of our water. But at the dispensary’s grow, out of 300 flowering plants, 30 varieties, only three varieties, 27 plants were affected. Unfortunately I was also using agricultural lime instead of dolomite lime at the time. When there is a disaster in your area such as a flood or forest fire check your waters pH often to prevent your own disaster with your plants.

So when to adjust soil pH ? The time to adjust soil pH is when you see a problem! The problem will be nutrient lock-out. You will not see this in the vegetating stage of your crop because your organic soil will be able to buffer the pH to a proper level. But as time goes on and watering has flushed out some of the buffers effectiveness, a pH problem is possible. This will usually happen in the flowering stage and the tell-tale signs are :” All growth stops ! The plants begin turning an ugly dull yellow. The flowers become miniaturized with the growth stoppage. The biggest problem : By the time you realize you have a problem with pH it will most likely be too late to save the plant. One reason for this is it’s very easy to misdiagnosis the problem at first and try other methods of correction. The first signs of pH nutrient lock-out mimic other deficiencies such as “nitrogen” deficiencies, “iron” and “magnesium” deficiencies. I have never been able to save a plant once I realized it had a pH problem.

Before Adjusting Soil pH What You Need To Know !

There are three measurements that are important to know before you attempt to correct a soils pH problem. The pH of your water supply, the pH of your water after adding any nutrient solution, fertilizer or supplements, and your soils pH value. Checking your water and nutrient solution is very easy with a good pH meter or a good test kit. Checking the pH of your soil isn’t as easy as it requires a little math.

Testing Your Soils pH !

You could buy a metal probe meter and use that if you want a ballpark figure, but for our purpose, that meter is not precise enough or accurate enough. I guarantee you will be disappointed with one of those meters for I have tried many manufactures products and found them all to be lacking on accuracy. If you are serious about growing organic marijuana I implore you to spend the money and buy a good pH meter. A good meter should cost at the minimum $30 USD, and most cost more, but are well worth the cost for their accuracy. Keep in mind you get what you pay for.

Two Ways to Test Soil.

The first way to test soil is to put some soil in a mason jar and add distilled water (its pH will be neutral) at a ratio of two parts water to one part soil. Shake it up and let it stand for a half hour or longer. Measure the pH of the water after this time has passed and all solids have settled. With organic soil you will have floaters of organic material so filtering the water thru a coffee filter would be a good idea. Check the pH with your meter. This method is difficult to utilize with potted plants because the plants roots get in the way of your soil sample and is best used before any plants are potted in it.

Testing Potted Plants – The Runoff Test

The second method is by measuring the water that runs out the bottom of the pot after watering a plant. This method requires that you first know what the pH of your water is that you are going to water your plant with. Once you know that pH number write it down. With a tray under the plant you are going to water, water the plant. Water the plant well untill the tray you put under the plant overflows. A tray an inch deep is perfect. Take a pH reading of the water in the tray. We call this runoff water and so we call this method of testing the Runoff Test. The color of the water should be clear to any shade of brown. You now must compare the two numbers to determine your soils pH. Lets say the water you put into the top of the pot had a ph of 7 , and the runoff water measured a 6.5. To calculate pH, subtract the ending pH number (6.5) from the starting pH number (7). This will give you .5. divide the .5 in half, giving you .25 and subtract that from the ending pH number6.5 equalling 6.25. In this case the soil pH will be about 6.25. This drop in pH from 7 pH water going in, to 6.5 pH water coming out the bottom is expected from organic soil. We always want to see a drop in pH in this test unless your water is very acidic, in which case we want to see a rise in pH. If the pH factor is higher in the runoff water than the water going into the top of the pot , then we have a real big problem with our soil being alkaline,and will need to correct it. If the runoff water comes out extremely acidic, lets say a 5pH and water in was a 7 pH, this would mean our soil is a 4 pH. Subtract 5 from the 7 giving you a 2. Take half of that (1 )and subtract that from the 5 giving your soil a pH of 4 which is very acidic.

How To Adjust The pH Of Organic Soil In A Potted Plant !

If you find your potted plants are having a pH problem that you must correct, correct the problem using water with the proper pH. Do not try to correct a potted plants pH problem by topdressing with lime or sulphur! Topdressing is when you add anything to the soil of a potted plant, such as supplemental nutrients ( earthworm castings, blood meal, magnesium sulphate, etc.) or sulphur or lime or any other substance. Some folks will tell you to take out the top inch or so of soil and put in powdered lime or sulphur, and then replace the soil you took out ; DO NOT DO THIS ! It’s true this method can work, but it’s not the best way and can cause more problems than it solves. The powdered lime or sulphur can filter into and plug up all the drainage in the pot. It can form an impassable layer in your soil where not even water can penetrate it. Water will sit at the top of the pot and pool and not drain thru the soil. I’ve been there and done that. Keep in mind that lime is used in cement as a catalyst to harden it. The same thing can happen in your pot. Another reason is that lime and sulphur does not devolve well and will be concentrated at the top roots and can cause damage to those roots. With the lime or sulphur at the top of the pot your pH will not be even throughout the pot with higher concentrations at the top and lower concentrations at the bottom of the pot. Please do not topdress to correct a pH problem.

The best way to correct a soil pH problem is by adjusting the pH of the water you use to water your plants with. This will assure even distribution of pH throughout the pot. Desolve the lime or sulphur in the water keeping track of the waters pH. You may also use pH up or pH down products if they are organic. To lower pH you may also use organic vinegaror organic lemon juice. Try and not change pH drastically in one application as a drastic change can do more harm than good. Change it slowly with your watering and only water when your plants are ready for water to prevent overwatering and root rot. I have never had to raise my waters pH but I do have to lower it on occasion and always when I do clones. I use organic vinegar, just drops at a time in a five gallon bucket of water. I use organic vinegar because it’s cheap, it’s easy, it goes a long way and it’s safe.

PH and Cloning !

Clones are very particular about water pH. If the waters pH you used in your rooting medium is too high or too low your clones will not root. The best pH for rooting clones will depend on your rooting medium. Rooting in soil pH should be around 6.5 to 7 pH. Rooting in a soilless mix, rockwool or sphagnum cubes pH is best between 5.5 and 6.5 pH. Adjust the waters pH prior to soaking the soilless medium, rockwool, sphagnum cubes, peat cubes etc. Keep watering your medium with the proper pH water as needed.

A word about Mycorrhizal Fungi and pH

Mycorrhizal fungiare the reason plants can live in soil with extremes of pH. Mycorrhizal fungi occur in all terrestrial environments including soils with varying pH from as low as 3 pH to as high as 9.5 pH. As you can imagine there are very few plants that can survive at the extremes of this range. Most plants grow in soils with pH of 4.5 to 8.0. While mycorrhizal fungi have preferences regarding soil pH, they are not absolute, and different species have different preferences. There is published research showing that plants and mycorrhizal fungi can modify the pH of their root environment. Bulk soil may be outside the preferred pH range for a plant , but the mycorrhizal fungi can surround the roots with a more tolerable pH environment. Some mycorrhizal fungi can produce a tightly woven sheath wrapped around a plants roots. The pH inside the sheath can be at or near the optimum pH for root growth compared to the pH of the bulk soil. This protection of the root by the fungal sheath can allow trees to grow in extreme places, such as abandoned coal mine areas.

The Bottom Line – pH and Marijuana

When growing organic marijuana, pH and marijuana go hand in hand. It is much easier to prevent a pH problem than it is to correct a pH problem. To prevent pH problems incorporate some Dolomitic lime into your soil mix before planting. Do this even with store-bought commercial organic soil.If you do this, you will prevent almost all pH problems from happening. I still suggest you check your waters supplys’ pH periodically, at least once a month and more often if forest fires or flooding is anywhere near where you are growing. The perfect soil pH for marijuana is 6.5, and a good range is 6 to 7 pH. Go to Page : Vegetating Marijuana !